7 replies to this topic

[babulala]

Pump is originally selected for pumping water at 30deg C, but due to changed process condition water temperature has increased to 75deg C, Now how pump perfromance form cavitation will be affected.

[Andree]

Pump is originally selected for pumping water at 30deg C, but due to changed process condition water temperature has increased to 75deg C, Now how pump perfromance form cavitation will be affected.

NPSH required will be higher - this increase will be approx.: (p_sat @ 70 degC) - (p_sat @ 30 degC)

[Padmakar Katre]

Dear,
Check the NPSHA i.e. Available and the NPSHR i.e. required. If the difference is there and which could manage to consider the effect of an increased Vap pressure of the water. Then there is no problem. Here I consider the only temp rise and not the flow.
Over and above to retain the an existing pump check is there any margin to increase the exsiting Vessel Elavation(BTL) or replace the suction line if you could find the major help in increasing the NPSH by reducing the suction line losses. Waiting for your comments. Another thing what you could do is to increse the sources pressure if it is easy to increase. Waiting for your reply.

[ankur2061]

Hi,

Recalculate your NPSHa with the new vapor pressure corresponding to 75 deg C . Check whether the difference between the recalculated NPSHa and the pump vendor supplied NPSHr is at least 1 m i.e. your new NPSHa is at least 1 m (3 ft) greater than NPSHr. This is a widely used rule of thumb for checking adequacy of NPSHa. If this difference is not available try the methods such as having a higher pressure in the suction source tank or decreasing the friction losses in the pipe by increasing the suction line size. The more difficult options would be increasing the height of the suction source tank or lowering the height of the pump.

Regards,
Ankur.

[Zauberberg]

Actually, by increasing water temperature it can be quite opposite - NPSH required would also drop, together with NPSH available drop due to increased vapor pressure:

http://www.mcnallyin...html/12-01.html

In your case, NPSHR would drop for approximately 0.1m (look at the chart below). I've seen this chart and the text from McNally institute long time ago, when I was searching for data related to BFW pumping - and I didn't have success in finding engineering, technical explanation for this phenomenon. It is connected to relative expansion rates of cold and hot fluid but, to be honest, I couldn't understand the concept behind this statement.

Maybe someone else can throw some light on this issue?

[attachment=968:NPSHR_Reduction.gif]

[joerd]

I found an interesting read at http://www.pumped101.com/#cavitation
The lower required NPSH at higher temperatures is, in my interpretation, that the vapor bubbles form and collapse at a slightly higher pressure (because the vapor-liquid equilibrium at a higher temperature corresponds to a higher pressure, this is just the boiling curve for water). Then, the volume difference between water vapor and liquid is less, which makes the "energy concentration" when the bubble collapses (cavitation) less severe.
For example, at 25 C the density of saturated water vapor (0.03 bar) is 0.023 kg/m3, while at 95 C the density (at 0.85 bar) is 0.50 kg/m3, which is a 22-fold increase. The density ratio of water vapor/liquid decreases from 43,000 to 1,900.

[Dexco]

There are some economical ways to correct Cavitation without rebuilding your plant.
1. For a few thousand $$ you can install a booster pump to up the inlet pressure.
2. If it is a multiplex plunger pump (triplex etc.) you can revamp the spring weights on the valves in the pump. Normally a lighter spring on the suction valves will compensate for the lower NPSH.
3. If you do not have knowledge of tuning a PD pump we cna supply Low NPSH valves for your pump. Then you do not have to worry about getting the springs correct.
4. Install a larger suction line, on non-boosted low NPSH systems you should aim for a velocity of 3 feet per second or slower thru the line. and 10 FPS on the discharge side. (These are optimum FPS rates)

Please let me know your pump make, model, plunger size, RPM, piping sizes/length, and a couple of pictures of the system and I can fix your problem via e-mail guaranteed.

For centrifugal pumps the best solutions are #'s 1 and 4.

Steven
pumps@dexcointernational.com
www.dexcointernational.com

[djack77494]

Two other strategies that may work in your situation:
1) Raise the liquid level in the source vessel, or
2) Raise the pressure in the source vessel.
These will not work in all situations, but may work in yours.